Overexpression and amplification of HER2 denotes a poor prognosis and outcome for breast cancer patients. Despite the availability of targeted anti-HER2 therapies, the clinical benefit of anti-HER2 agents is short-lived. We have made the unexpected observation that HER2+ human BC cell line HCC1954 exhibits a switch from aerobic glycolysis to oxidative phosphorylation metabolism as resistance to the anti-HER2 kinase inhibitor, lapatinib, sets in. Additionally, we have found that the dual EGFR/HER2 kinase inhibitor lapatinib lengthens the cell cycle time of HER2+ HCC1954 breast cancer cells without increasing apoptosis or cell death. This lengthening of the cell cycle correlates to reduced proliferation and population doublings under lapatinib treatment, as well as metabolic changes. Upon generating lapatinib-resistant HER2+ breast cancer cells, we discovered that lapatinib resistance conferred sensitivity to the metabolic inhibitor phenformin. These data suggest that HER2+ breast cancer cells may have adapted altered ErbB signaling networks to overcome anti- HER2 treatment and use alternative metabolic strategies to fuel uncontrolled proliferation. The goal of this application is to investigate HER2+ breast cancers switch from glycolytic to oxidative metabolism in response to lapatinib treatment, and its relationship to resistance. We will use paired parental and drug-resistant cell lines to assess the metabolic effect of targeted anti-HER2 treatment on a functional and molecular level with single cell resolution. Furthermore, the population dynamics of sensitive and resistant cells treated with anti- HER2 and phenformin will be interrogated using acute and chronic animal models. We believe that lapatinib resistant cells will be more sensitive to phenformin in vivo, and the use of phenformin will reduce tumor burden. Our approach includes an assessment of drug treatment perturbations at a cellular level, complemented with a more focused analysis of the altered redox metabolism at a molecular level. Collectively, we hope to gain a better understanding of the mechanism of anti-HER2 resistance, probe the feasibility of metabolic inhibitors as anti-cancer therapy, and recapitulate drug action and response. If we are successful, our results may open the door to novel combination therapy including lapatinib and metabolic inhibitors such as metformin, which may improve performance of anti-HER2 targeted therapy and inhibit resistance.